Sure no one invests in bitcoin or in any other kind of an investment with an intention of losing money, yet part of the way of assuring an ability to stick with an investment through the ups and the downs is to use extra money that you don't need for expenses.
Sure, there is no amount of money we can afford to lose, but the reason why people say they should invest the amount we can afford to lose is because of the risk that comes with bitcoin investment. There is the possibility of losing money, so we would rather use a small portion of the money we have in bitcoin investment because of the longevity.
You seem to be describing the concept of investing an amount that we can afford to lose in a strange way, since when you invest into bitcoin, then you likely have a 4-10 year or longer timeline for when you are considering that you are going to want to start to get at the money, yet in the meantime, the are always possibilities that bitcoin could go to zero during that time. Accordingly, you have to be willing to figure out an amount of money that you accept that it could go to zero.
That is the same with other investments that you make, there are possibilities that each of your various investments (if you have investments other than bitcoin and dollars) could go to zero, and the scenarios in which they could go to zero are different depending upon what kind of investment asset that you have.
We calculate and we expect profits in our various investments, and surely some investments do better than others, and if we think about bitcoin as an asymmetric bet in which the most that we could lose is 100%, then we should account for that along with the various upside scenarios and also the various sideways scenarios... and surely we could calculate the various scenarios and figure out how much we are willing to put into bitcoin even while knowing that there are scenarios that involve bitcoin possibly going down or even going to zero. We can choose our BTC position size and how we get into bitcoin based on such assessments. Frequently people suggest that bitcoin is amongst the best, if not the best of investments available, so in some sense, we can understand and appreciate that there could be scenarios that guys choose to invest into bitcoin in quite modest ways, yet their modest investment amount end up paying off quite stupendously based on how much bitcoin ended up appreciating in value.. and so historically, we already have a lot of examples of such great price appreciation circumstances that left those quite well off who invested into bitcoin, even those who invested relatively modestly into bitcoin.
So yeah, past results do not guarantee future results, and also we know that there have been folks who invested into bitcoin and who lost all of their money, and frequently those had been the traders and/or those using leverage, or alternatively leaving bitcoin on exchanges or otherwise not securing private keys. In other words, we have examples of folks losing money on bitcoin when those who just invested straight into bitcoin and did not trade and did not leverage and who errored on the side of BTC accumulation and HODL, were able to profit stupendously, sometimes more than 100x their investment amount... but it still could well end up taking time to play out depending upon when they got into bitcoin and including that sometimes it could also take years and years to establish a bitcoin position with various purchases spread out over various times in which the BTC was bought.
My point still is that even if bitcoin has a lot of upside scenarios, each of us still have to figure out a bitcoin position size in which we are comfortable with how much value we put into bitcoin, whether we buy in lump sum, or DCA over the years, or buying on dips or some combination of those strategies. We should not be taking for granted or expecting that our investment is guaranteed to go up even though the reason that we invest in bitcoin is out of an expectation that its chances of going up are greater than its chances of going down and also that we likely reasonably expect that some of the various upside scenarios are so great that it can still end up paying off quite stupendously to have relatively modest amounts of value in bitcoin, even though never guaranteed that the great upside scenarios will end up playing out. It never has been guaranteed that bitcoin prices would go up in price and/or in value, even though historically there have been quite a few rich folks who have come about from being involved in bitcoin and mostly stacking their bitcoin and/or holding it, and it is also quite likely that bitcoin is ongoingly facilitating the largest peaceful wealth transfer that has ever happened from the no coiners to the coiners.